The present invention relates to a boom assembly and more specifically to a boom assembly in which a section of the boom assembly has sloping top and bottom walls which tend to maximize the overall stability, strength and stiffness of the boom section.
A known crane is provided with boom sections having a generally rectangular cross sectional configuration, in a manner similar to that disclosed in U.S. Pat. No. 3,690,742. This rectangular boom section is provided with horizontal and vertical slider pads to enable the boom section to withstand both vertical and sideward loading. However, difficulty has been encountered in keeping the vertical pads in contact with both sides of the boom section simultaneously since manufacturing tolerances result in the production of booms which do not have exactly constant widths throughout their length. Any sideways motion of one boom section inside of another adds a twisting moment to the load, decreases the overall elastic stability of the boom, and makes an operator's job of precisely positioning a load more difficult. Of course, any wear of a vertical slide pad increases the aforementioned problems.
Another known boom assembly has a generally square cross sectional configuration with slider pads which engage corner sections of the boom in the manner disclosed in U.S. Pat. No. 3,830,376. This boom construction results in relatively high slider pad loading. In addition, the boom section has a square cross-sectional configuration so that the lines of action of the slider pad forces are through the center of the boom section. This results in marginal lateral stability when the boom section is subjected to a vertical main load.
Still another known boom construction is disclosed in U.S. Pat. No. 3,481,490. One of the boom sections of this construction has peaked top wall on which slider pads, i.e. rollers, are mounted. The peaked top wall of this boom section has an enclosed peak angle of 90.degree. so that the sections of the top wall slope at an angle of 45.degree. to a horizontal plane. Due to the relatively large angle of slope of the various sections of the top wall, the peak of the top wall of this known boom section is located a relatively large distance from the central axis about which the boom section is loaded. Therefore, relatively large stresses will be present at the peak of the top wall of the boom section upon loading of the boom. The bottom wall of this known boom section is generally flat and is not peaked. Since the bottom wall of the boom section is loaded in compression, the unpeaked or generally flat configuration of the bottom wall tends to promote a buckling of the bottom wall when it is subjected to relatively large compression forces resulting from the application of a relatively large load to the boom section. In addition, it should be noted that lateral stability of the boom section is impaired since the top slider pad forces intersect above the center of the boom section while the bottom slider pad forces intersect below the center of the boom section.